Metallic wool ignition materials



corporation of Delaware No Drawing. Filed Nov. 14, 1960, Ser. No. 68,668

(til. 149-5) 13 Claims.

This invention relates to a device for the ignition of solid rocketpropellants.

In the field of rocket propulsion, solid rocket propellants usually havea plastic-like, caked appearance and burn on their exposed surfaces. Thephysical mass or body of the propellant is referred to as the grain andthe ignition of the combustion surface of the propellant grain isaccomplished by means of a pyrotechnic igniter, which in turn is usuallystarted by means of electrical current or percussion action. Ideally,the ignition device should supply uniform heat at high temperatureinstantaneously over the entire uninhibited surface of the grain.

It is an object of the present invention to supply uniform heat at hightemperature instantaneously and with negligible shock to the entireuninhibited surface of a solid rocket propellant grain.

We have found that an igniter consisting of a metallic wool, the termwool being used hereinafter to define broadly that physical statewherein a metal has been fabricated into fine metallic threads such asthe well known steel wool, an adhesive, and a solid oxidizer can beprepared, which, when activated by an electric current or other suitablemeans, burns instantaneously with uniform heat and no shock. The termshock is used herein to describe an explosion or detonation. Further,this igniter meets the requirements of being safe handling with maximumdependability.

The process for the preparation of the igniter consists of taking ametallic wool made of a metal such as magnesium, aluminum, zirconium,iron, steel, etc., dipping it into a liquid adhesive which remains tackyafter drying, and thereafter bringing the adhesive-covered metal- =licwool into contact with a powdered oxidizer in such a manner that theoxidizer, because of the adhesive, adheres to the threads of themetallic Wool. That the igniter thus prepared is stable and safe tohandle has been proven by its ability to remain inert when placed in anoven for 2 hours at 120 C.

While, in general, most metallic wools burn when in the presence of anoxidizer, some burn faster than others. Thus, though a wool made of ironwill function within the scope of the present invention, magnesium hasproven to be superior for igniter purposes due to its faster burningrate and is the preferred metallic Wool. The adhesive can be of a typesuch that a uniform layer, having the appropriate *tackiness for holdingthe desired amount of oxidizer, can be applied by solution dipping orother means. For obvious reasons, the adhesive must be inert, that is,unreactive at ordinary temperatures with both the oxidizer and themetallic wool. Certain low molecular weight resins and condensationproducts have proven satisfactory for the purpose. These adhesives wepreferably hydrophobic so that atmospheric water vapor is not absorbedin the oxidizer thereby starting corrosion and thus deactivating themetal wool. Sui-table adhesives include low molecular weightpolyisobutylenes, polypropylenes, polyisoprenes, and polyacrylics, andalso certain polyglycols. Atactic polypropylene glycol in particular hasgiven excellent results.

The powdered oxidizer can be selected from those well known to therocket art such as ammonium perchlorate and lithium perchlorate. Forbest oxidation, the amount 3,06,804 Patented Apr. 23, 1963 of oxidizershould be stoichiometrically equivalent to the amount of metallic wool.However, it should be realized that the wool will burn in air at somerate without the oxidizer and increasing the oxidizer up to thestoichiometric amount is for the purpose of obtaining the maximumburning rate and assuring optimum combustion in a closed chamber whereatmospheric oxygen is not available. Without the oxidizer, the burningis sporadic and slow, and when an excess of oxidizer is used, the excessconsumes heat needed for ignition of the grain.

When igniter function includes the generation of gaseous pressure in therocket combustion chamber, a friable organic material may be blendedWith the oxidizer and the blend applied to the metallic Wool. Organicmaterials best suited for this purpose are those containing largeproportions of gas-generating agents, for example, carbon for making COand CO nitrogen for N and nitrogen oxides, hydrogen for E 0, andespecially oxygen to avoid using excessive amounts of solid oxidizer. Itis useful for this organic material also to contain high energylinkagessuch :as acetylenic bonds and the like with the exception that linkagesand groups contributing :to excessive instability or corrosiveproperties necessarily must be avoided. Materials suitable for thepurpose include hydrophobic solid pclyalkylene oxides, for example,polypropylene oxides of molecular weight in the approximate range10,000-25,000.

The following specific examples of our invention:

illustrate the practice Example I A pad of fine magnesium wool weighing3 grams was lightly packed into the desired shape. The pad was lightlypacked in order to leave voids within the structure. This pad was thendipped into an adhesive consisting of 2.0 weight percent atacticpolypropylene glycol of approximately 20,000 molecular weight dissolvedin normal hexane, shaken to remove excess adhesive, air dried until thehexane was essentially evaporated, and thereafter placed into acontainer containing three times the pads weight of ground ammoniumperchlorate. The container was shaken at such a rate that the ammoniumperchlorate penetrated throughout the pad. The pad was removed from thecontainer and excess ammonium perchlorate not adhering to the pad wasshaken off. The pad was prepared for activation by connecting two finecopper wires to the pad at separate points in such a manner as to havegood electrical contact. Activation was accomplished by connecting thelead wires from v. potential to the two th-in copper wire leads andthereafter subjecting the pad to 110 v. When so activated it burnedsubstantially instantaneously without detonation.

Example 11 A pad was prepared as in Example I using magnesium wool andlithium perchlorate. Its burning properties were similar to those of theproduct of Example 1.

Example III A pad was prepared as in Example I using steel wool andammonium perchlorate. When activated it burned smoothly but not asrapidly as pads from magnesium or aluminum.

We claim:

1. A device for the ignition of solid rocket propellants which comprisesa loosely packed mass of metallic wool, fabricated from a metalcombustible in air in this form, and coated throughout with about thestoichiometric quantity of a powdered solid inorganic oxidizing salt,said oxidizing salt being held to the metallic wool by [an inertadhesive, and the mass of metallic wool being connected to appropriatemeans for activation.

2. The device of claim 1 wherein the oxidizing salt is an inorganicperchlorate.

3. The device of claim 2 wherein the metallic wool is magnesium wool.

4. The device of claim 2 wherein the metallic wool is aluminum wool.

5. The device of claim 2 iron wool.

6. The device of claim 2 wherein the metallic wool is zirconium wool.

7. A pad of metallic wool, fabricated from a metal combustible in air inthis form, coated throughout with about the stoichiometric quantity of apowdered solid inorganic oxidizing salt, said oxidizing salt being heldto the metallic wool by an inert adhesive.

8. A process for making a device as described in claim 1, which process'compn'ses coating a loosely packed mass of metallic Wool fabricatedfrom a metal combustible in :air in this form with an inert adhesive,and thereafter bringing the adhesive-covered metallic wool into contactwith a powdered solid inorganic oxidizing wherein the metallic wool issalt, thereby coating said wool with about the stoichiometric quantityof oxidizing salt.

9. A process as described in claim 8 wherein the oxidizing salt is aninorganic perchlorate.

10. A process as described in claim 9 metallic wool is magnesium wool.

11. A process as described in claim 9 metallic wool is aluminum wool.

12. A process as described in claim 9 metallic wool is iron wool.

13. A process as described in claim 9 metallic Wool is zirconium wool.

wherein the wherein the wherein the wherein uhe References Cited in thefile of this patent UNITED STATES PATENTS 812,195 Escales Feb. 13, 19062,037,101 Van Liempt Apr. 14, 1936 2,959,001 Porter Nov. 8, 1960 202,973,713 Burton Mar. 7, 1961

1. A DEVICE FOR THE IGNITION OF SOLID ROCKET PROPELLANTS WHICH COMPRISESA LOOSELY PACKED MASS OF METALLIC WOOL, FABRICATED FROM A METALCOMBUSTIBLE IN AIR IN THIS FORM, AND COATED THROUGHOUT WITH ABOUT THESTOICHIOMETRIC QUANTITY OF A POWDERED SOLID INORGANIC OXIDIZING SALT,SAID OXIDIZING SALT BEING HELD TO THE METALLIC WOOL BY AN INERTADHESIVE, AND MASS OF METALLIC WOOL BEING CONNECTED TO APPROPRIATE MEANSFOR ACTIVATION.